Design challenges and common issues when developing websites with AJAX

After the main HTML document has loaded, AJAX loads content from the server and replaces parts of the document with that content rather than reload the main document. It’s as simple as that. AJAX stands for “Asynchronous JavaScript and XML” and was meant to load only XML documents, but we soon used it to load everything under the sun, and so the XML part was quickly forgotten. The asynchronous part is the killer feature; but what is it?